Definition of mental arithmetic in English:

noun

Daniel Tammet is a man with an exceptional mind: he can perform mental arithmetic faster than a pocket calculator, and learn a new language fluently, even one as tortuously complex as Icelandic, in a matter of days.

Lawrence Lee was known in the local area to be very good at mental arithmetic and it was this love of mathematics that he passed on to his daughter.

Calling himself a mathemagician, he astonishes audiences with amazing feats of mental arithmetic.